# std::free

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< cpp‎ | memory‎ | c

Defined in header `<cstdlib>` |  |
void free( void* ptr ); |  |

Deallocates the space previously allocated by std::malloc, std::calloc, std::aligned_alloc(since C++17), or std::realloc.

If ptr is a null pointer, the function does nothing.

The behavior is undefined if the value of ptr does not equal a value returned earlier by std::malloc, std::calloc, std::aligned_alloc(since C++17), or std::realloc.

The behavior is undefined if the memory area referred to by ptr has already been deallocated, that is, `std::free` or std::realloc has already been called with ptr as the argument and no calls to std::malloc, std::calloc, std::aligned_alloc(since C++17), or std::realloc resulted in a pointer equal to ptr afterwards.

The behavior is undefined if after `std::free` returns, an access is made through the pointer ptr (unless another allocation function happened to result in a pointer value equal to ptr).

The following functions are required to be thread-safe: The library versions of `operator new` and `operator delete` User replacement versions of global `operator new` and `operator delete` std::calloc, std::malloc, std::realloc, std::aligned_alloc(since C++17), std::free Calls to these functions that allocate or deallocate a particular unit of storage occur in a single total order, and each such deallocation call happens-before the next allocation (if any) in this order. | (since C++11)

### Parameters

ptr | - | pointer to the memory to deallocate

### Return value

(none)

### Notes

The function accepts (and does nothing with) the null pointer to reduce the amount of special-casing. Whether allocation succeeds or not, the pointer returned by an allocation function can be passed to `std::free`.

### Example

Run this code

    #include <cstdlib>

    int main()
    {
        int* p1 = (int*)std::malloc(10 * sizeof *p1);
        std::free(p1); // every allocated pointer must be freed

        int* p2 = (int*)std::calloc(10, sizeof *p2);
        int* p3 = (int*)std::realloc(p2, 1000 * sizeof *p3);
        if (!p3) // p3 null means realloc failed and p2 must be freed.
            std::free(p2);
        std::free(p3); // p3 can be freed whether or not it is null.
    }

### See also

C documentation for free

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